Gas turbines for power generation systems must satisfy the highest demands with respect to reliability, power, efficiency, economy, and operating service life. Modern high-efficiency combustion turbines have firing temperatures that exceed about 2,300° F. (1,260° C.), and firing temperatures continue to increase as demand for more efficient engines continues. Many components that form the combustor and “hot gas path” turbine sections are directly exposed to aggressive hot combustion gases. The use of coatings on turbine components such as combustors, combustion liners, combustion transition pieces, combustion hardware, blades (buckets), vanes (nozzles) and shrouds is important in commercial gas turbine engines.
Coatings, such thermal barrier coating systems, contribute to desirable performance characteristics and operating capabilities at elevated temperatures. Typical thermal barrier coating systems include a bond coating disposed on the substrate of the turbine component, and a thermally insulating top coating disposed on the bond coating. The bond coating can be a diffusion type of coating or an MCrAlY type of coating, where M is nickel, cobalt, iron, or a combination thereof. The bond coating provides oxidation and hot corrosion protection to the underlying substrate of the turbine component, which are typically made of nickel-based, cobalt-based, or iron-based superalloys. Bond coatings also provide an interface for the ceramic top coat to adhere. However, while bond coatings improve adherence of the thermally insulating top coating, bond coatings are themselves often weaker than the underlying substrate due to their chemical compositions and phase constituents, which are different than the composition of the substrate, and may also be different than the phase constituents of the substrate. The chemical mismatch between the substrate and the bond coating creates a diffusion couple, and results in the formation of an interdiffusion zone with properties that are different than the bond coating and the substrate, and also creates a differential in the coefficient of thermal expansion across the substrate-interdiffusion zone-bond coating. It furthermore results in the formation of new, sometimes detrimental phases, which may cause embrittlement at the substrate/bond coating interface. Among other factors, the quality of the bond coating/substrate interface and the coefficient of thermal expansion transition in the thermal barrier coating system has an impact on the overall life capability of the thermal barrier coating system, and may lead to early spallation of the thermally insulating top coating.